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Full text of "The manufacture and properties of iron and steel"

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STEEL' CASTINGS.                                         413
is only attained by long experience combined with specialized knowledge."
Some engineers specify that the cavities shall not exceed a certain percentage of the total -area, but the common-sense method is to clothe the inspector with discretionary power,, for a flaw may be harmless on the under surface of a base-plate when it would be fatal in the rim of a wheel. It should be noted that there is a radical difference between' a "blow-hole" and a "pipe." The cavities often seen where the "sink-head," or "riser," is cut off are not evidence of unsoundness, but exactly the opposite, for they show that feeding continued after the riser was exhausted, and that the interior has been rendered solid at the expense of the surface.
SEC. XXd.—Phosplioms and sulphur in steel castings.—In specifications for steel castings, the important point is to state that phosphorus shall not exceed .04 per cent. Other elements may be guarded against by requiring a proper ductility, but phosphorus is often masked by other factors, and manifests itself only in that brittleness under shock which is its inherent characteristic. This is an important matter in the case of rolled metal, but is of more vital moment in steel castings, for these generally fail by sudden strain and shock.
The content of sulphur is of little importance', for it affects the cold properties very slightly, but it will do no harm to specify that it shall not be over .05 per cent., good castings generally containing less than this proportion. Copper need not be mentioned, for there is no evidence that it has any influence upon the finished casting.
SEC. XXe.—Effect of silicon, manganese and aluminum.—The elements used to procure solidity are silicon, manganese and aluminum. Their value to the steelmaker is due in great measure to their power of uniting with oxygen, the -action being as follows:
8.44 parts manganese unite with 1.00 part of oxygen: 8.44 parts aluminum unite with 8.01 parts of oxygen. 8.44 parts silicon, unite -with 3.93 parts of oxygen.
Hence the aluminum is three times, and the silicon four times, as efficient as manganese, weight for weight, while they have an additional value from their greater affinity for oxygen, since this enables them to seize the last traces from the iron and wash the bath so much the cleaner,